I was replying to >note that at-will employment simply lacks several (the big one being "certainty"). In particular a "contract" that says "you can do whatever

I was replying to >note that at-will employment simply lacks several (the big one being "certainty"). In particular a "contract" that says "you can do whatever" is clearly not a contract under common law. Which was in reply to a comment about every job having a contract. Further your other examples could be contractually enforced. If you accepted an employment contract based on the compensation plan, that forms part of the contract. Raises are updates to the contract, unless you're saying it's potentially ok to claw back 40 years of pay raises when some one retires. Offer letters are probably more questionable but if there's a specific start date and pay has already been discussed, it seems to me that that would form a contract. The key thing in this particular situation seems to be that there was an error, and the error was rectified quickly.